Hitherto, aluminum sheets have been used as a support for a lithographic printing plate. A roughening process of the surface of the support, i.e., so-called graining process, is conducted to improve adherence between the support and a light-sensitive layer and to provide non-image areas with water retentivity.
Examples of the aforesaid graining process include mechanical roughening processes such as sandblast, ball graining, wire graining, brush graining with a nylon brush and an abrasive/water slurry, and a liquid honing (e.g. jetting of a high pressure abrasive/water slurry to the surface), and chemical roughening processes such as surface roughening treatment with etching agents, for instance alkali, acid or a mixture thereof. Further, an electro-chemical graining process as described in Japanese Patent Application (OPI) No. 146234/79 (the term "OPI" as used herein refers to a "published unexamined patent application") and Japanese Patent Application (OPI) No., 28123/73, such as a process of a combination of mechanical graining and electrochemical graining as described in Japanese Patent Application (OPI) 123204/83 and a process of a combination of mechanical graining and chemical graining with an aqueous saturated solution of an aluminum salt of mineral acid as described in U.S. Pat. No. 4,242,417.
Aluminum plates which have been subjected to the aforesaid roughening treatment may be used as a support for a lithographic printing plate as such or after further chemical treatment. However, in the case where a lithographic printing plate is desired to have the high printing durability, anodic oxidation treatment is further carried out to enhance adhesion of images to the support and to raise mechanical strength of the surface of non-image areas.
In the field of the production of supports for printing plates, the anodic oxidation treatment is performed by applying a direct or alternating current to an aluminum sheet in an aqueous or non-aqueous solution of sulfuric acid, phosphoric acid, chromic acid, oxalic acid, sulfamic acid, boric acid, benzenesulfonic acid or a combination of two or more of these acids.
As mentioned above, the anodic oxidation treatment is carried out using various electrolytic solutions. Above all, sulfuric acid and phosphoric acid are commonly used.
A presensitized plate from which a lithographic printing plate is to be prepared is subjected to a plate-making process comprising imagewise exposure, development and application of desensitizing gum. The resulting printing plate is then mounted to a printing machine to perform printing. Sometimes, scratches are formed on the printing plate during transfer or plate-making or at the time when dust is removed from the printing face during printing. In the case where such scratches are present on non-image areas, ink will deposit on the parts of scratches during printing, which will then cause stains in the form of the scratches on a printed matter.
When the surface of the non-image areas is treated by anodic oxidation as stated above to create an alumina coating on the surface, the surface becomes hard and is not easily scratched. Thus, appearance of scratch-form stains is prevented. This effect may be enhanced by increasing the amount of the anodic oxidation coating. However, the increase of the amount of the anodic oxidation coating requires a lot of electric power, which results in the increase of production costs.